Automatic telephone system



A ril 4, 1939- R. PFANNSCHMIDT AUTOMATIC TELEPHONE SYSTEM Filed April 27, 1936 IN VENTOR. PFAN NSCHM l DT ATTORNEY.

Patented Apr. 4, 1939 UNITED STATES PATENT OFFICE AUTOMATIC TELEPHONE SYSTEM Application April 27, 1936, Serial No. 76,663 In Germany April 30, 1935 12 Claims.

The present invention relates to a circuit arrangement for automatically operated telephone systems in which impulses for setting the selectors are transmitted over the telephone speak- 5 ing lines by means of alternating current. In particular it relates to an arrangement in which dialling takes place over the subscribers line by means of alternating current. This manner of impulse transmission is known to be particularly advantageous in the case of badly insulated subscribers lines such as are frequently met with in rural telephone systems.

The present invention has the advantage over the known arrangement that it permits electric isolation of the lines. By selecting a corresponding transmission ratio for the isolating transformer the alternating potential necessary for a satisfactory transmission. of impulses can always be attained.

The invention is characterized in that the impedance of a repeater occurring in the subscribers line over which the impulse receiving relay is influenced in a local alternating current circuit by the impulse is continuously varied during the impulse transmission.

The attached drawing represents an embodiment of the invention, wherein Figs. 1 to 6 show various modifications of the invention.

For the sake of simplicity only the impulse 30 contact nsi of a subscribers station in an automatic telephone system is shown in Figs. 1-6. The subscribers stations are connected with the exchange Z by way of the subscribers lines TL. In Fig. 1 the subscribers line is isolated by a repeater Dr, the secondary winding 11 of the repeater occurring in the line while the primary winding I is arranged in a local circuit in series with the impulse receiving relay R and a source WQ of alternating current. As long as the impulse contact nsi is open at the subscribers station the repeater Dr acts as a transformer having its secondary winding open while when the impulse contact net is closed the repeater Dr represents a transformer having its secondary winding short-circuited. Now it is known that in the case of a transformer having its secondary winding open the impedance in the primary circuit is high and the current flowing in the circuit is therefore small. Accordingly the impulse receiving relay does not energize when the impulse contact is open. When the secondary winding of the repeater Dr is short-circuited by contact nsz' the impedance in the primary circuit is suddenly diminished. The current in this circuit, therefore, increases and the relay energizes.

Sincethe closing and opening of the secondary circuit takes place intermittently, relay R. also energizes intermittently. As is shown by dotted lines in Fig. 1 a battery may also be connected in one of the subscribers lines in the exchange for the purpose of premagnetizing the secondary winding of the repeater. The mode of operation remains, however, fundamentally the same as that without the use of a battery.

The arrangement shown in Fig. 1 has the diaadvantage that in the case of bad insulation it only allows a relatively weak shunt between the leads of the subscribers line. In this respect the arrangement shown in Fig. 2 is considerably better. rangement consists in that a glow lamp G1 is connected in series with the impulse receiving relay R and the primary winding of the repeater. Moreover, in parallel with the impulse receiving relay and the glow lamp an impedance (in the embodiment a condenser C0) is connected up. If the subscribers loop is open at the contact nsi and the impedance in the primary circuit of the repeater is accordingly large the fall of potential between points I and 2 in the primary circuit is small on account of the weak current, so that the potential difference. between points I and 2 is also small. When the impedance is decreased by the closing of the impulse contact the potential difference between points I and 2 in the primary circuit on account of the increased voltage drop becomes sufliciently large to exceed the striking potential of the glow lamp GZ. The glow lamp strikes and relay R can now energize in series with the glow lamp. Naturally, an arrangement is also possible in which the relay already energized is deenergized over the operated glow lamp and is again energized when the glow lamp goes out. This arrangement can be so adjusted by corresponding adjustment of the individual switching means that only a potential drop of one volt is necessary for striking the glow lamp. Thus even when a considerable shunt exists between the two leads of the subscribers line corresponding variations in the impedance of the repeater which produce a sufficient potential at the point 2 for striking the glow lamp G1 are produced on the closing and opening of the impulse contact at the subscribers station.

While Fig. 1 shows an arrangement in which the impulse receiving relay R is arranged in series with the source of alternating current and the primary winding of the repeater, Fig. 3 shows an arrangement in which the impulse receiving The manner of operation of this arrelay R is arranged in parallel with the primary winding of the transformer isolating the subscribers line. The mode of operation is quite clear. Relay R in this case energzes on the opening of the impulse contact nsi since the impedance of the primary winding of the repeater which is arranged in parallel is high, and relay R releases on the closing of the contact nsi because in this case the primary winding of the repeater constitutes a small impedance in parallel.

In Fig. 4 the repeater isolating the subscribers line is formed by the impulse receiving relay R itself. Relay R is provided with two windings of which one is arranged in series with the subscribers line while the other, inductively coupled to the first is connected to the alternating current source WQ. As long as the impulse contact nsi is closed, the current generated in the lefthand winding of relay R, due to the flux created by the current flowing in the right-hand winding of the relay, is approximately 180 out of phase with the current flowing in the right-hand winding. Accordingly, the flux created by the current flow generated in the left-hand winding neutralizes the flux created by the current flowing in the right-hand Winding, thereby preventing energization of the relay. When the impulse contact 1181' is open the relay energizes, because no opposing current flow can be generated in the left-hand winding of the relay. Accordingly, relay R energizes over the circuit including the right-hand winding each time contact nsi is opened and restores each time contact nsi is closed.

Figs. 5 and 6 show an arrangement in which the primary windings of'the repeater isolating the subscribers line are applied as inductances in special circuits. The secondary windings of the inductances are arranged in the subscribers loop. On the impulse transmission they are alternately opened and short circuited, whereby the tuning of'the tuned circuits is continuously varied. The alternating current operates the impulse receiving relay R over these tuned circuits. Since these circuits are tuned to the frequency of the alternating current source the impulse receiving relay is intermittently operated according to each variation away from and back to the tuned condition of the circuits throughout the impulse transmission.

Fig. 7 shows the application of the invention to.

a so-called call finder connection. Only those details relevant to the invention are represented. The subscribers station Tst. has local battery feeding. The initiation of a call on the part of the station is effected by magneto ringing. On operating the magnet Ind winding I of relay R in the exchange is energized. Relay R locks up in the following circuit: earth, contacts 4t and Sr winding II of relay R, winding of relay An, battery and earth. Relay An starts up the call finder AS in the known way. When the call finder has reached the calling line relay T energizes in the following circuit: earth, wiper c of the call finder, contact Ir, winding of relay T, battery and earth. Relay T energizes and locks up in a testing circuit over its contact 5t independently of relay R. Relay T by opening contact 4t breaks the holding circuit for relay R and the starting relay An. By opening contact It and closing contacts 2t and 31? the subscribers line is switched through to the exchange while disconnecting winding I of relay R. The subscribers line is isolated by the speech repeater U62 in the exchange At the point of symmetry of the two primary windings of this repeater the secondary winding II of the repeater Dr is connected. By way of the primary winding of this latter repeater an arrangement is coupled which is fundamentally that illustrated in Fig. 2 but with the difference that, instead of a pure impedance in parallelwith the impulse relay J and the glow lamp GZ, it possesses an ohmic resistance Wi. The alternating current circuit in this arrangement is closed by the testing relay T associated with the subscribers line at contact 925 after the testing of the call finder on the calling subscribers line.

After the calling subscriber has taken up his receiver whereby the microphone circuit is closed by s'witchhook contacts I-IU3, the bell W is disconnected from the subscribers line by switchhook contacts HUI and HU2, and the lower speaking lead has been switched through, dialling can commence. on dialling earth is applied intermittently to the two speaking leads simultaneously over the impulse contact nsi and dial off normal contact nsa and thereby the secondary circuit of the repeater Dr is alternately short circuited and opened. The dial oiI normal contact nsa is closed as long as the dial is out of the rest position. I

The impulse receiving relay J is alternately closed and deenergized during the impulse transmission. On energizing it completes a circuit over contact I07 for the repeating relay H. The latter at contact 8h breaks the impulse loop set up after the setting of the call finder AS on the subscribers selector'and thereby repeats the impulses.

On completing the dialling, the impulse receiving relay finally deenergizes. The alternating current from the source WQ remains connected up to the subscribers line over transformer Dr, however, throughout the whole call.

When after terminating a call the calling subscriber replaces his receiver, the switchhook HUS breaks the microphone circuit. By closing switchhook contacts HUI and HU2 the bell W, earthed at the middle of its winding, is connected as a bridge over the speaking leads. The secondary winding II of the repeater D1 is thereby again, and this time finally, short-circuited. The impulse receiving'relay J energizes and energizes relay H. The latter relay by means of its contact 872. breaks the line circuit passing to the subsequently arranged selector, for example, a group selector, for a long time. At the same time, however, the testing circuit passing over the c wiper of the call finder is broken. The testing relay T releases and thereupon all the circuits are returned to their original condition.

Fig. 8 shows an arrangement in which the secondary winding of the repeater Dr is divided into two symmetrical halves which are connected in series with the two windings of the speech repeater and conveniently between the two. Earth is connected to the point of symmetry of the windings. This arrangement has the advantage over the arrangement shown in Fig. '7 that the leakagefrom the subscribers line occurring due to bad insulation only affects the operation of the repeater to half the previous extent. In addition this arrangement has the advantage over that shown in Fig. 7 in that the stray currents which are produced by the asymmetric leakage from'the two leads of the subscribers line on the application of the current are only effective to half the previous extent.

'each time the impedance thereof is varied by said impulse transmission,

2. In a repeating system, a line, a local circuit including a source of alternating current, a transformer having its secondary winding connected 1 to said line and its primary winding connected in said local circuit, impulse contacts on said line for alternately closing and opening the circuit of said secondary winding during dialling thereby alternately diminishing and increasing the impedance in said local circuit including said primary winding, and an impulse responding relay also included in said local circuit operated each time the impedance thereof is changed by the operation of said impulse contacts.

3. In a repeating system as claimed in claim 1 in which the impulse responding relay is connected in multiple with the primary winding of said transformer in said local circuit.

4. In a repeating system, a line, means for transmitting impulses over said line, a local circuit including a source of alternating current, an impulse responding relay having a first winding and a second winding, said first winding terminating said line and said second winding included in said local circuit, said first and second windings being inductively coupled for varying the impedance in said local circuit in response to each impulse transmission thereby operating said relay in accordance with the impulses transmitted.

5. In a repeating system, a line, means for transmitting impulses over said line, a tuned resonant circuit including a source of alternating current, a transformer having its secondary e Winding terminating said line and its primary winding included in said tuned circuit to form the inductive element thereof, the impedance in said tuned circuit varied in response to each impulse transmission for changing the tuning of said tuned circuit,.and an impulse responding relay included in said tuned circuit operated for each change in the tuned circuit.

6. In a repeating system, a line, means for transmitting impulses over said line, a local circuit including an impedance element and a source of alternating current, a transformer having its secondary winding terminating said line and its primary winding included in said local circuit for varying the total impedance of the local circuit in response to each transmitted impulse, a glow lamp connected in shunt of said impedance element in said local circuit, said glow lamp alternately striking and extinguishing by potential change in response" to the varying of the total impedance in said local circuit by said impulse transmission.

7. In a repeating system, a line, means for transmitting impulses over said line, a local circuit including an impedance element and a source of alternating current, a transformer having its secondary Winding terminating said line and its primary Winding included in said local circuit for varying the total impedance of the local circuit in response to each transmitted impulse, a glow lamp in series with an impulse responding relay connected in shunt of said impedance element in said local circuit, said glow lamp alternately rendered conductive and non-conductive by potential change in response to the varying of the total impedancein said local circuit by said impulse transmission, said relay operated each time said lamp is rendered conductive and released each time said lamp is rendered non-conducting.

8. In a telephone system, a line, a repeater for said line, a local circuit including a source of alternating current, a transformer having its secondary winding connected to the mid-point of said repeater and its primary winding included in said local circuit, an impulse contact on said line for transmitting impulses over the two line conductors of said line through said repeater to the secondary winding of said transformer, the impedance in said local circuit varied in response to each impulse transmitted, and an impulse responding relay included in said local circuit operated for each change in impedance in said local circuit.

9. In a telephone system, a line, a repeater for said line, means responsive to a call on said line for connecting said repeater to the two line conductors of said line, a normally open circuit including a source of alternating current, said circuit closed when said lineis connected to said repeater, a transformer having its secondary winding connected to the mid-point of said repeater and its primary winding included in said circuit, an impulse contact on said line for transmitting impulses over the two conductors of said. line through said repeater to said secondary Winding of, said transformer, the impedance in said circuit varied in response to each impulse transmitted, and an impulse responding relay included in said circuit operated by each change in'impedance in said circuit.

10. In a telephone system, a line, a finder switch for finding said line and extending a connection in response to a call on said line, a normally open circuit including a source of current, said circuit closed in response to said finder finding said line, a transformer having its secondary Winding connected to said line and its primary winding included in said circuit, an impulse contact on said line for transmitting impulses over said line to said secondary winding, the impedance in said circuit varied in response to each impulse transmitted, and an impulse re sponding relay included in said circuit operated by each change in impedance in said circuit.

11. In a telephone system, a line, a finder switch for finding said line and extending a connection in response to a call on said line, a normally open circuit including a source of current, said circuit closed in response to said finder finding said line, a transformer having its secondary winding connected to said line and its primary winding included in said circuit, an impulse contact on said line for transmitting impulses over said line to said secondary winding, the impedance in said circuit varied in response to each impulse transmitted, an impulse responding relay included in said circuit operated by each change in impedance in said circuit, and means for again varying the impedance of said circuit to operate said relay on termination of the call for releasing said connection.

12. In a telephone system, a line, a finder switch for finding said line and extending a connection in response to a call on said line, a normally open circuit including a source of current,

said circuit closed in response to said finder finding said line, a transformer having its secondary winding connected to said line and its primary Winding included in said circuit, an impulse contact on said line for transmitting impulses over said line to said second winding, the impedance in said circuit varied in response to each impulse transmitted, an impulse responding relay included in said circuit operated by each change in impedance in said circuit, means for again varying the impedance of said circuit to operate said relay on termination of the call for releasing said connection, and means responsive thereto for opening said circuit thereby disconnecting the inductive connection of said source from said line.

RUDOLF PFAN'NSCHMIDI. 

